1. Field of the Invention
The present invention relates to a mobile communication unit for performing a high-quality information transmission by allocating a plurality of channel numbers to one user.
1. Description of the Related Art
In recent years, a code division multiple access mobile communication unit has been developed for practical application, as described in the literature, "On the System Design Aspects of Code Division Multiple Access (CDMA) Applied to Digital Cellular and Personal Communications Networks, (May 19-22, 1991, IEEE Vehicular Technology Conference)". The configuration of the prior-art technique of the code division multiple access mobile communication unit is shown in FIG. 1. In FIG. 1, a symbol 1 designates a set of units at the transmitter side such as the base station or the like and 2 designates a set of units such as a car telephone set and a portable telephone set at the receiver side. Symbols 3, 4 and 5 designate information input circuits provided corresponding to channel numbers allocated to each user for the set of units at the transmitter side, to which information of the respective users are inputted. An information input circuit 3 corresponds to #1, an information input circuit 4 corresponds to #2, and an information input circuit 5 corresponds to #m. Symbols 6, 7 and 8 designate spread modulators connected to the information input circuits 3, 4 and 5 respectively, for carrying out a spread processing in a spread code corresponding to each channel number. A symbol 9 designates a combining unit for transmitting spread signals of a plurality of users by combining these spread signals together. A symbol 10 designates a despreader for carrying out a despread processing in the spread code of the channel allocated to each user in the sets of units at the receiver side 2. In the set of units at the transmitter side 1, parameters W1(t), W2(t), - - -, and Wm(t) are inputted as orthogonal spread codes and a parameter PN(t) is inputted as a pseudo-noise series, to the spread modulators 6, 7 and 8, respectively. By multiplying these orthogonal spread codes by the pseudo-noise series, spread codes S1(t), S2(t), - - -, and Sm(t) corresponding to the respective channels are obtained, and spread processing is carried out in these spread codes. In the following explanation, the above pseudo-noise series will be referred to as "PN series". In the set of units at the receiver side 2, each unit has the despreader 10. When the channel number of the receiver side units shown in FIG. 1 is #i, a parameter Wi(t) is inputted as an orthogonal spread code and a parameter PN(t) is inputted as a PN series, to the despreader 10, respectively, so that a despread processing is carried out in the spread code corresponding to this channel. FIG. 2 shows an example of the spread codes to be used corresponding to the channel numbers allocated to the users in a certain cell for carrying out the spread and despread processings described above.
In the mobile communication unit having the above-described structure, when user information has been inputted from the information input circuits 3, 4 and 5 at a predetermined information transmission bit rate, for example, B [bps], a spread processing is carried out by the spread modulators 6, 7 and 8 in the spread code corresponding to the channel numbers allocated to the users. Then, the combiner 9 combines the spread signals of the users and transmits the combined result. In the mean time, when the combined spread signal has been received by the units at the receiver side 2, the despreader 10 carries out a despread processing by using the spread code corresponding to the channel number allocated to the user, so that the information is reproduced at the information transmission bit rate B [bps] and is then outputted from an information output circuit 11.
FIGS. 3 to 5 show the status of changes in the waveform of a signal when the signal transmitted as user information at a certain information transmission bit rate is processed, transmitted and despreaded. The user information is inputted from the information input circuits 3, 4 and 5 in the form of a spectrum signal 12 having a band width B and a power spectrum density P as shown in FIG. 3. When the spectrum signal 12 has been spread processed by the spread modulators 6, 7 and 8, the power within the band width B is distributed in a spread band width S of a spread multiple spectrum on the circuit shown in FIG. 4 to form a spread signal 13 as shown in FIG. 4. The spread modulators 6, 7 and 8 correspond to the channel numbers allocated to the users and the spread codes are set to the respective channel numbers at different values as shown in FIG. 2. Therefore, the spread signal 13 takes a multiple structure with different values between channels. FIG. 4 shows an example of a four-channel spread multiple spectrum.
In the unit at the receiver side 2, the spread signal 13 is despread processed in Wi(t) for the orthogonal spread code and PN(t) for the PN series. Therefore, of the four-channel spread multiple spectrum, the spread signal of the channel corresponding to the spread code Si(t), that is, the power of a desired wave, is concentrated to the band width B again, and the multiple other user signals (three channel components) take the spreaded waveform. The waveform in the spreaded status remains as an interference wave. When a filtering is applied to pass the band area B in the unit at the receiver side 2, a spectrum of a desired wave 14 and an interference wave 15 after the despread is obtained as shown in FIG. 5. When an SIR (a signal to interference ratio) which is the ratio of the power of the desired wave 14 to the power of the interference wave 15 takes a predetermined value, required communication quality can be maintained.
When B=9600, that is, the information transmission bit rate is 9600 bps, at most 64 channels can be set within the range in which the SIR can secure a predetermined value, from the-viewpoint of an interference ratio. In this respect, there is an example that 64 kinds of Walsh code have been used as orthogonal spread code.
According to the above-described prior-art mobile communication unit, however, there has been a problem that the information transmission bit rate for one user that can be obtained is only not higher than the information transmission bit rate which corresponds to the value obtained by dividing the chip bit rate of the spread code by the spread rate, so that user information that is transmitted at a high information transmission bit rate can not be transmitted.